The present invention generally relates to automated material handling systems and, more particularly, to systems and methods for managing test material in automated material handling systems.
Automated material handling systems are used in a variety of industries to move various materials from one location to a another location. Semiconductor fabrication facilities, in particular, commonly employ automated material handling systems for fabricating integrated circuits on semiconductor wafers.
A conventional semiconductor fabrication plant typically includes multiple fabrication areas or bays interconnected by a path, such as a conveyor belt. Each bay generally includes the requisite fabrication tools (interconnected by a subpath) to process semiconductor wafers for a particular purpose, such as photolithography, chemical-mechanical polishing, or chemical vapor deposition, for example. Material stockers or stocking tools generally lie about the plant and store semiconductor wafers waiting to be processed. The wafers are usually stored in cassettes each of which typically hold up to 25 wafers. Each material stocker typically services two or more bays and can hold hundreds of cassettes.
The semiconductor fabrication plant, including the bays, material stockers and the interconnecting path, typically operates under control of a distributed computer system running a factory management program, such as WorkStream Open sold by Consilium, Inc. In this environment, the automated process specification. Manufacturers commonly employ non-production wafers or test wafers to facilitate the fabrication of production wafers. These test wafers may be used for a number of different purposes. For instance, qualification test wafers may be used by a tool (e.g., a deposition chamber) prior to processing production wafers with the tool to calibrate the tool and/or to ensure the operability of the tool. As another example, dummy wafers may be used to fill slots within a carrier or chamber where a xe2x80x9cfullxe2x80x9d state is required for proper processing.
As a result of the large number of wafers being fabricated and the numerous process steps performed on each wafer, a large number of test wafers and thus a large number of cassettes are typically employed during fabrication. As a facility can only operate with a finite number of cassettes, test cassettes take capacity away from production cassettes, thereby reducing the number of production wafers which may be fabricated over a period of time. Throughput is further diminished as a result of the manner in which the cassettes are occupied. In a typical facility, each cassette typically holds a single type of test wafer, the cassette being designated for a particular procedure (e.g., qualification, particle count, etc.). In addition, on average a typically cassette holds 15 or less test wafers, far less than its capacity. This adds to the number of test cassettes and further contributes to throughput inefficiencies.
Semiconductor manufactures compete in a highly competitive and capital-intensive industry. A state-of-the-art semiconductor fabrication plant typically includes hundreds of different fabrication tools and can cost $1 billion or more. New plants can also become obsolete relatively quickly as the dimensions of semiconductor devices decrease. Consequently, to generate higher revenues, semiconductor manufactures continually seek to increase to the throughput and yield of semiconductor wafers and find systems and techniques which increase either of these parameters to be highly desirable.
The present invention generally provides techniques for managing test material (e.g., test wafers) in automated material handling systems. These techniques may, for example, significantly reduce the capacity of test cassettes and/or reduce the number of test cassettes needed for a given number of products. This can increase the throughput of material through these systems.
In accordance with an embodiment of the invention, test material is classified into a plurality of classes. A time profile for each class of test material for a production period is determined. The test material is then placed into cassettes based on the determined time profiles for each class.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures in the detailed description which follow more particularly exemplify these embodiments.